The well-known technique of electrospray ionization is used in mass spectrometry to produce ions. In conventional electrospray ionization, a liquid is pushed through a very small charged capillary. This liquid contains the analyte to be studied dissolved in a large amount of solvent, which is usually more volatile than the analyte. The conventional electrospray process involves breaking the meniscus of a charged liquid formed at the end of the capillary tube into fine droplets using an electric field. The electric field induced between the electrode and the conducting liquid initially causes a Taylor cone to form at the tip of the tube where the field becomes concentrated. Fluctuations cause the cone tip to break up into fine droplets which are sprayed, under the influence of the electric field, into a chamber at atmospheric pressure, optionally in the presence of drying gases. The optionally heated drying gas causes the solvent in the droplets to evaporate. According to a generally accepted theory, as the droplets shrink, the charge concentration in the droplets increases. Eventually, the repulsive force between ions with like charges exceeds the cohesive forces and the ions are ejected (desorbed) into the gas phase. The ions are attracted to and pass through a capillary or sampling orifice into the mass analyzer.
Incomplete droplet evaporation and ion desolvation can cause high levels of background counts in mass spectra, thus causing interference in the detection and quantification of analytes present in low concentration. It has been observed that smaller initial electrospray droplets tend to be more readily evaporated and, further, that droplet sizes decrease with decreasing flow rate. Thus, it is desirable to reduce the flow rate and, consequently, the droplet size, as much as possible in order to obtain mass spectra with minimal background interference. Nano-electrospray, with flow rates per emitter in the range of less than several hundred nanoliters per minute to 1 nanoliter per minute, has been found to yield very good results, in this regard. Further, it has been found that the efficiency of ionization is much higher in nanospray mode and that the response is more linear than in other spray modes. For instance, Ficcaro et al., in a technical paper titled “Improved Electrospray Ionization Efficiency Compensates for Diminished Chromatic Resolution and Enables Proteomics Analysis of Tyrosine Signaling in Embryonic Stem Cells” (Analytical Chemistry 81, 2009, pp. 3440-3447), demonstrate that, in the assessment of LCMS performance, the improved electrospray ionization efficiency at low flow rates outweighs deterioration of chromatographic separation, even at chromatographic flow rates below Van Deemter minima. However, conventional electrospray devices and conventional liquid chromatography apparatuses which deliver eluent to such electrospray devices are typically associated with flow rates of several microliters per minute up to 1 ml per minute.
Attempts have been made to manufacture an electrospray device which produces nanoelectrospray. For example, Wilm and Mann, Anal. Chem. 1996, 68, 1-8 describes the process of electrospray from fused silica capillaries drawn to an inner diameter of 2-4 μm at flow rates of 20 nl/min. Specifically, a nanoelectrospray at 20 nl/min was achieved from a 2 μm inner diameter and 5 μm outer diameter pulled fused-silica capillary with 600-700 V at a distance of 1-2 mm from the ion-sampling orifice of an Atmospheric Pressure Ionization mass spectrometer. Other nano-electrospray devices have been fabricated from substantially planar substrates with microfabrication techniques that have been borrowed from the electronics industry and microelectromechanical systems (MEMS), such as chemical vapor deposition, molecular beam epitaxy, photolithography, chemical etching, dry etching (reactive ion etching and deep reactive ion etching), molding, laser ablation, etc.
In order to realize the aforementioned benefits of nano-electrospray at higher overall flow rates, electrospray arrays of densely packed tubes or nozzles have been developed, using either capillary pulling or microfabrication and MEMS techniques, so as to increase the overall flow rate without affecting the size of the ejected droplets. For example, FIG. 1 illustrates an array of fused-silica capillary nano-electrospray ionization emitters arranged in a circular geometry, as taught in United States Patent Application Publication 2009/0230296 A1, in the names of Kelly et al. Each nano-electrospray ionization emitter 2 comprises a fused silica capillary having a tapered tip 3. As taught in United States Patent Application Publication 2009/0230296 A1, the tapered tips can be formed either by traditional pulling techniques or by chemical etching and the radial arrays can be fabricated by passing approximately 6 cm lengths of fused silica capillaries through holes in one or more discs 1. The holes in the disc or discs may be placed at the desired radial distance and inter-emitter spacing and two such discs can be separated to cause the capillaries to run parallel to one another at the tips of the nano-electrospray ionization emitters and the portions leading thereto.
FIGS. 2A-2B show, respectively, a schematic view of one electrospray system and a cross-sectional view of an electrospray device of the system, as taught in United States Patent Application Publication 2002/0158027 A1, in the name of Moon et al. The electrospray device 4 generally comprises a silicon substrate or microchip or wafer 5 defining a channel 6 through substrate 5 between an entrance orifice 7 on an injection surface 8 and a nozzle 9 on an ejection surface 10. The nozzle 9 has an inner and an outer diameter and is defined by a recessed region 11. The region 11 is recessed from the ejection surface 10, extends outwardly from the nozzle 9 and may be annular. The tip of the nozzle 9 does not extend beyond the ejection surface 10 to thereby protect the nozzle 9 from accidental breakage.
A grid-plane region 12 of the ejection surface 10 is exterior to the nozzle 9 and to the recessed region 11 and may provide a surface on which a layer of conductive material 14 including a conductive electrode 15 may be formed for the application of an electric potential to the substrate 5 to modify the electric field pattern between the ejection surface 10, including the nozzle tip 9, and the extracting electrode 54. Alternatively, the conductive electrode may be provided on the injection surface 8 (not shown).
The electrospray device 4 further comprises a layer of silicon dioxide 13 over the surfaces of the substrate 5 through which the electrode 15 is in contact with the substrate 5 either on the ejection surface 10 or on the injection surface 8. The silicon dioxide 13 formed on the walls of the channel 6 electrically isolates a fluid therein from the silicon substrate 5 and thus allows for the independent application and sustenance of different electrical potentials to the fluid in the channel 6 and to the silicon substrate 5. Alternatively, the substrate 5 can be controlled to the same electrical potential as the fluid.
As shown in FIG. 2A, to generate an electrospray, fluid may be delivered to the entrance orifice 7 of the electrospray device 4 by, for example, a capillary 16 or micropipette. The fluid is subjected to a potential voltage Vfluid via a wire (not shown) positioned in the capillary 16 or in the channel 6 or via an electrode (not shown) provided on the injection surface 8 and isolated from the surrounding surface region and the substrate 5. A potential voltage Vsubstrate may also be applied to the electrode 4 on the grid-plane 12, the magnitude of which is preferably adjustable for optimization of the electrospray characteristics. The fluid flows through the channel 6 and exits or is ejected from the nozzle 9 in the form of very fine, highly charged fluidic droplets 18. The extracting electrode 17 may be held at a potential voltage Vextract such that the electrospray is drawn toward the extracting electrode 17 under the influence of an electric field.
All presently known nano-electrospray array devices utilize a conventional delivery method in which analyte-bearing liquid is delivered to a hollow nozzle by means of micro-capillaries or micro-tubes, so as to be emitted from an interior bore of the nozzle. There are many limitations to the use of such small-bore capillaries and nozzles, such as clogging, difficulty in producing a spray and, in the case of silica capillaries, difficult handling. Furthermore, with such conventional electrospray delivery techniques, an increase in salt concentration results in spraying difficulty and there is a sudden decline in desorption efficiency of ions into the gaseous phase. Accordingly, such delivery methods cannot be applied to NaCl aqueous solutions on the order of 150 mM, such as physiological saline solution.